Example magnetograms derived from MDI data (left panel) and HMI data (right panel) on 30 March 2011. The overlying red contours indicate the border of the masked areas determined by a threshold value of 30 G (left panel) and 20 G (right panel), inside which the data points are not used in the travel-time measurements. The MDI data on this day shown here is only for comparison since HMI data are used instead of MDI data after 1 May 2010 in this work.
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